INTRODUCTION: We present the reliability of ultra-high field T2* MRI at 7T, as part of the UK7T Network's``Travelling Heads"study . T2*-weighted MRI images can be processed to produce quantitative susceptibility maps (QSM) and R2* maps . These reflect iron and myelin concentrations, which are altered in many pathophysiological processes . The relaxation parameters of human brain tissue are such that R2* mapping and QSM show particularly strong gains in contrast-to-noise ratio at ultra-high field (7T) vs clinical field strengths (1.5 - 3T). We aimed to determine the inter-subject and inter-site reproducibility of QSM and R2* mapping at 7T, in readiness for future multi-site clinical studies .
METHODS: Ten healthy volunteers were scanned with harmonised single- and multi-echo T2*-weighted gradient echo pulse sequences . Participants were scanned five times at each``home"site and once at each of four other sites . The five sites had 1x Philips , 2x Siemens Magnetom, and 2x Siemens Terra scanners . QSM and R2* maps were computed with the Multi-Scale Dipole Inversion (MSDI) algorithm (https: //github.com/fil-physics/Publication-Code). Results were assessed in relevant subcortical and cortical regions of interest (ROIs) defined manually or by the MNI152 standard space .
DISCUSSION: Mean susceptibility (χ) and R2* values agreed broadly with literature values in all ROIs . The inter-site within-subject standard deviation was 0.001 - 0.005 ppm (χ) and 0.0005 - 0.001 ms-1 (R2*). For χ this is 2.1-4.8 fold better than 3T reports, and 1.1-3.4 fold better for R2* . The median ICC from within- and cross-site R2* data was 0.98 and 0.91, respectively . Multi-echo QSM had greater variability vs single-echo QSM especially in areas with large B0 inhomogeneity such as the inferior frontal cortex . Across sites, R2* values were more consistent than QSM in subcortical structures due to differences in B0-shimming . On a between-subject level, our measured χ and R2* cross-site variance is comparable to within-site variance in the literature, suggesting that it is reasonable to pool data across sites using our harmonised protocol .
CONCLUSION: The harmonized UK7T protocol and pipeline delivers on average a 3-fold improvement in the coefficient of reproducibility for QSM and R2* at 7T compared to previous reports of multi-site reproducibility at 3T . These protocols are ready for use in multi-site clinical studies at 7T.